This application is based on application No. 11-276407 filed in Japan, the contents of which is hereby incorporated by reference.
1. Field of the Invention
The present invention pertains to a fusing device used in an electrophotographic image forming apparatus such as a copying machine, printer or facsimile machine.
2. Description of the Related Art
An electrophotographic image forming apparatus such as a copying machine, printer or facsimile machine has a fusing device that fuses onto a sheet a toner image carried on the sheet.
While various methods may be used by the fusing device, in response to the recent demand for energy conservation, fusing devices using the induction heating method, which offers a better conversion efficiency than a fusing device using a halogen lamp as the heat source, have been proposed in Japanese Laid-Open Patent Application Hei 7-287471, for example.
A fusing device using the induction heating method comprises a hollow conductive member that is fixed or movable, a closed magnetic circuit iron core that forms a closed magnetic circuit and part of which runs through the empty space of the conductive member, and an inductive coil that is wrapped in a spiral fashion around the closed magnetic circuit iron core. A magnetic flux generates induction heat in the conductive member by supplying electric current to the inductive coil, so that the conductive member is heated based on induction heating.
The conductive member has a roller configuration. A sheet carrying a non-fused toner image is conveyed by this roller-configured conductive member and another roller that is in contact with the conductive member while the sheet is grasped in between them, and fusing is performed based on the heat caused by induction heating and the pressure exerted by the rollers.
Incidentally, one of the features of the induction heating method is that the temperature rises rapidly. In other words, as an electric current is supplied to the coil, a temperature appropriate for fusing is reached in a very short period of time, such as one or two seconds. In order to take advantage of this strength, a material having a small heat capacity is usually used for the conductive member that is heated based on induction heating. Conversely, it may be said this fusing device easily cools down to such an extent that its heat capacity is small.
In particular, the end areas of the roller easily cool down in comparison with the center area of the roller. Therefore, in a conventional fusing device using the induction heating method, the temperature is increased over the entire roller so that the temperature of the end areas does not decrease below the level appropriate for fusing. Consequently, the temperature of the end areas is lower than the temperature of the center area. The problem therefore occurs that the temperature along the length of the roller, i.e., the temperature along the width of the sheet, is not uniform. In addition, because the temperature in the center area is higher than the level appropriate for fusing, an excessive amount of energy is consumed accordingly, which is not desirable.
In view of the situation described above, the object of the present invention is to provide an improved induction heating fusing device.
Another object of the present invention is to provide an induction heating fusing device that can fuse a toner image using a uniform fusing temperature throughout the width of the sheet.
In order to attain these and other objects, according to one aspect of the present invention, the induction heating fusing device comprises a core that forms a magnetic circuit, an inductive coil located around the core, and a continuous conductive member into which part of the core extends, wherein the diameter of the inductive coil varies from the center of the length of the core to the ends of the core.
Furthermore, the diameter of the inductive coil is larger at the center of the length of the core than at one end of the core.
Furthermore, the inductive coil is alternately connected at one end of the core.
Furthermore, the diameter of the coil at the other end is the same as at the center of the core.